Spinal motor neurons play a critical role in the neural control of movement. Motor neurons differentiate soon after formation of the neural tube and project axons along stereotyped pathways to innervate selected muscle targets. Although there is considerable information on the mechanisms that control later aspects of motor neuron development almost nothing is known about the steps involved in determining the identity of motor neurons. The overall alm of the proposal is to define the cellular interactions and molecules that control the identity of motor neurons. To approach this problem we will identify transcription factors that are expressed at early stages of motor neuron differentiation and will determine the function of these genes. The cellular interactions that control the identity and diversity of motor neurons will also be examined using these genes as markers. Our preliminary studies have shown that members of the LlM family of homeodomain proteins are expressed at early stages of motor neuron differentiation. Based on these studies, the proposal has three major aims: l. To characterize LlM homeodomain proteins as markers of functional subsets of motor neuron. 2. To define the cellular interactions that generate motor neuron diversity using LlM homeodomain proteins as markers. 3. To use in vitro assays to address the function of LIM homeodomain proteins in motor neuron differentiation. The extent and specificity of LIM homeodomain expression in motor neurons will be addressed by isolation of LlM homeodomain genes, the generation of antibodies against their encoded proteins and the localization of mRNA and protein by in situ hybridization and immunocytochemistry. Retrograde labelling techniques will be used to relate LlM homeodomain gene expression and specific motor neuron subgroups that project to defined targed muscles. The cellular interactions that control motor neuron diversity will focus initially on a subset of lateral motor column neurons for which definitive markers are available. In vivo manipulations in chick embryos and in vitro assays of motor neuron differentiation in neural plate explants will determine the existence and examine the source and biochemical nature of signaling molecules that induce lateral motor column neurons. The involvement of LIM homeodomain proteins in the inductive pathway leading to motor neuron differentiation will be assessed by antisense oiigonucleotide ablation of LlM homeodomain protein expression and by introducing these genes into neural plate cells in vitro. In the long term, these studies will be important in defining and interpreting the underlying causes of a wide variety of neurological disorders that result in the degeneration and death of motor neurons.